418 STORAGE SYSTEM 



B. STORAGE-TISSUES. 



1. Tissues serviwj for the storage of plastic materials. 



In the tissues which serve for the reception of plastic substances, 

 nitrogenous and non-nitrogenous compounds are deposited in the most 

 varied combinations and proportions. The only further general state- 

 ments that can be made with regard to this point are, first, that the 

 non-nitrogenous reserve-materials usually exceed the nitrogenous sub- 

 stances in amount ; and, secondly, that the various carbohydrates 

 appear to be mutually interchangeable, while in certain cases they are 

 altogether replaced by fats. 



The equal value, from a nutritive point of view, of the various carbo- 

 hydrates and fats is a point to which a little more consideration must be 

 devoted ; this equivalence may be deduced not only from the chemical 

 changes attendant upon the germination of seeds, but also from a study 

 of the comparative anatomy of storage-tissues. It frequently happens, 

 for example, that different storage-organs of the same plant do not 

 contain the same non-nitrogenous reserve-materials ; in the same way 

 totally different non-nitrogenous compounds may occur, in homologous 

 storage-organs, in closely related species. Whereas, for instance, starch 

 is stored in potatoes, inuline in the tubers of Dahlia and cane-sugar 

 in Beet-root, the seeds of the two former plants contain oil, while those 

 of the Beet are starchy. The grains of most Grasses contain starch ; 

 in a few instances, however, fatty oil is present instead {Phraij mites 

 communis, Koeleria cristata, etc.). In the case of Impatiens Balsam inn, 

 again, amyloid is stored in the cotyledons, in the form of enormously 

 thickened cell-walls ; in other species of Impatiens the cotyledonary 

 tissue is quite thin-walled, and the place of reserve-cellulose is taken 

 by oil. Similarly the bulb-scales of the Onion contain a large amount 

 of glucose, whereas the Tulip and many other Liliaceae accumulate 

 starch in their bulbs. 



From a purely physiological point of view 7 , starch and oil are 

 interchangeable ; in given circumstances, however, one or the other 

 may be preferable for ecological reasons. Fats contain a far higher 

 proportion of carbon than starch or any other carbohydrate. Triolein, 

 for instance [C 3 H 5 (OC 18 II 33 0)3, the chief constituent of Olive oil], 

 contains 7 7 '4 per cent, by weight of carbon, as compared with the 

 4 4 "4 per cent, of this element contained in the molecule of starch 

 [(C 6 H 10 O 5 ) x ]. Since the specific gravity of starch is 1/56, while that 

 of the fats lies between "91 and '96, a given volume of starch contains 

 approximately the same quantity of carbon as an equal volume of fat, 

 but is about 1*7 times as heavy as the latter. Fat thus represents a 



